Propagate dlt column descriptions to Iceberg docs

[nicosuave]

Summary:

• propagate dlt column description metadata to Iceberg NestedField docs during schema conversion
• carry docs through added columns and update existing docs only when a non-null incoming description differs
• preserve existing Iceberg comments when descriptions are absent or unchanged

Testing:

• uv run pytest tests/test_schema_converter.py tests/test_schema_evolution.py -q
• uv run pytest -q (173 passed, 15 skipped)